Low-temperature polysilicon (LTPS), which is used in liquid crystal display devices, are widely used in current small-to-medium-sized display devices, as it has a high carrier mobility. In the field of organic EL display devices, too, the development of an array step has been advanced on the basis of LTPS technology.
However, it is difficult to form a sufficiently uniform LTPS layer in an excimer laser anneal (ELA) step. Variations in the characteristics of thin-film transistors (TFTs) due to nonuniformity in LTPS cause nonuniformity in the luminance of the organic EL display devices.
To address this problem, measures to reduce variations in the characteristics of the TFTs have been taken by forming a correcting circuit in a peripheral circuit or a pixel. Further, such measures have been taken in the ELA step as to perform laser irradiation over and over multiple times. However, measures such as these pose problems in terms of apparatus cost, material cost of a laser, and the like.
To address this problem, not only transistors made of polycrystalline silicon that are believed to be high in driving capability but also transistors using a transparent amorphous oxide semiconductor that are expected to be small in characteristic variation have been studied to reduce power consumption and take measures against variations in the characteristics of the transistors.
For example, Japanese Unexamined Patent Application Publication No. 2015-225104 discloses a display device including two or more transistors in one pixel, wherein the two or more transistors include a first transistor whose channel semiconductor layer is made of polycrystalline silicon and a second transistor whose channel semiconductor layer is made of an oxide semiconductor.